1) Field of the Invention
The present invention relates to a turbine. In particular, but not exclusively, the present invention relates to a turbine such as a turbine for a drilling assembly; a drilling assembly including a turbine; and an at least partly polymeric rotor and stator for use in a turbine.
2) Description of Related Art
In the field of, for example, oilfield exploration, “Turbo-drilling” is an established method of creating a “bore-hole” by drilling through geological strata. Turbo-drilling machines, as the descriptive name implies, are turbines powered by either an “impulse” or “reaction” type turbine blade system. Impulse type systems are ones which are driven by a fluid at atmospheric pressure, whilst reaction type systems are ones driven by fluid pressurised to above atmospheric pressure, possessing energy which is partly kinetic and partly pressure.
Further, turbine blade systems of the reaction type are ones where the blade profile of the stator and rotor is essentially an aerodynamic profile, subject to the “Bernoulli principle” of different pressure being created by a fluid passing over two opposite exterior surfaces of a common body. Where these surface lengths are different, this creates areas of high pressure on one surface and low pressure on the opposite surface, this creating a pressure differential which results in a movement of the body towards the low pressure side of the body.
This principle is used in turbo-drills to transfer the hydraulic power of a drilling fluid being pumped through the turbine system of stators and rotors into rotational power of a rotor element, which is rigidly attached to a drive shaft system, and ultimately connected to a drilling bit (which may be one of various designs and configurations) for the explicit purpose of fracturing a rock structure. The drilling fluid having exited the drilling machine is further utilised for the removal of drilling cuttings by being transported in suspension to the surface up the annulus of the borehole for disposal.
The art of generating power from a turbine system is well known and there are many forms of turbine systems being employed in various engineering fields. In these various fields, there are problems associated with blade life, for example, degradation of the aerofoil shape of the turbine blade leading to reduced efficiency of the blade system.
In the field of oilwell drilling, the majority of drilling is accomplished with the aid of a drilling fluid, typically mud (as noted above), air or more recently foam, this fluid being utilised for control of the well bore and transportation of rock cuttings. Drilling mud is a suspension of barites in an oil or water based solution of various densities. Drilling foam is generally used in under-balanced drilling applications normally associated with high velocity flow systems, whilst air is used in high speed drilling applications not normally associated with oilwell drilling.
In all of these drilling fluid systems there is an associated abrasive characteristic of the fluid. This abrasiveness gradually degrades the internal components of the drilling machine, which abrasive wear is known as “erosion”, where the rate of erosion is related to fluid velocity, drilling fluid density and characteristics of component molecular structures. Generally speaking, high fluid velocities are characteristic of any fluid flowing through a turbine blade system in operation. Steel components are also subjected to “corrosion”, related to the chemical composition of the drilling fluid and turbine component molecular structures. This erosion and corrosion can lead to reduced efficiency of a turbine blade system and reduce the life of a turbine drilling system.
Traditionally, the method of manufacture of turbine rotors and stators for oilwell drilling applications is to manufacture the components from steel of various compositions, for example, carbon steels or stainless steels. These steel materials have certain advantages and inherent disadvantages, the main advantage being that the complex shape of the blade profile is readily cast by various methods, for example, the “lost wax” method, or more recently with the introduction of CNC machine tools, the steel can be machined; however this is a costly and time consuming process. Also steels of certain chemical composition can be heat treated to enhance the end product characteristics. Stator and rotor elements are typically constructed as a one piece cast/moulded component or made up from several constituent parts, such as rotor blade hubs, stator blade shrouds and blades.
The selected method of manufacture is generally dependent on the turbine application. In the manufacture of oilwell drilling turbine stators and rotors, the traditional method of manufacture has generally been to produce one piece castings in a steel material for the stator and rotor components, by a casting and finish machining process. The stators and rotors are normally mounted in a tubular body with a drive shaft component, these components being secured by various means to effect a rotation of rotor elements on a drive shaft. Securing of the stator and rotor components can be achieved by mechanical compression or keying systems.
Presently known systems therefore suffer from various disadvantages related to the construction of the systems and limitations of their use due to, in particular, erosion and/or corrosion of components in use.
It is amongst the objects of embodiments of the present invention to obviate or mitigate at least one of the foregoing disadvantages.